You can easily extend your Iranian visa by its original duration at least twice. This means a 30-day visa with 2 extensions gives you 90 days in Iran.

Extensions can be issued from the day you turn up at the visa renewal office, or after the end date of your visa, depending on the official. The extension is often issued in 1 day, but it can also take longer.

The cost of a visa renewal is 300,000 Rials (around $9-10). You need your passport, 2 photocopies of the photo page, of the current Iran visa, and of the entry stamp.

Bandar Abbas

No hassle extension.

Esfahan

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Mashad

The immigration office of Mashhad is located next to Azad University, Rahnamaee St.

ShirazUsed to be a hassle-free place to extend visa until recent reports (starting from February 2015) about problems there. They either insist on visas to be extended the day before they expire, or refuse to extend for more than 1 week, or just sent people to Esfahan or Tehran.

Tabriz

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Tehran

The immigration office is on Parvin Boulevard, roughly a 5 minute walk south of Tehran Pars metro station, take the left exit. Procedure is easy and straightforward. You can avoid queueing in the bank by purchasing the ready bank receipt from a guy outside the office for a small extra fee. You will need to pay a nearby Melli Bank 300,000 rial for the extension. Get the bank numbers first from the alien affairs office, and after paying the bank come back with the bank slip. Office official can help with filling in the application form. They say that you can photocopy and get passport photos taken at the alien affairs office in Tehran for a price. Once your paperwork is all stapled together, the office manager will then sign off on your application either on the spot, or on a particular day. P/S They moved office of allien affairs in Tehran in 2014 to Pars Boulevard

YazdThe immigration office is located near Abouzar Sq on Kashani Street. Opens at 7:30 a.m. They extend for 30 days, usually on the spot at the same day. You will need a copy of the passport. copy of the visa with an entry stamp, plus 2 pictures. Payment can be done nearby at Melli Bank on the same square. Staff is friendly there, you can apply for extension with many days still left in your visa.